The invention concerns a process for the production of acyl cyanides by reaction of carboxylic acid halides with metal cyanides at elevated temperature. Acyl cyanides are important intermediate products for example for the production of .alpha.-ketocarboxylic acids.
It is known that acyl cyanides can be produced by reacting a metal cyanide with a carboxylic acid halide. However, the previously known processes have considerable disadvantages.
Thus, in the production of aliphatic acyl cyanides there must be employed acid bromides since the reactability of the acid chloride is not sufficient. For example, acetyl chloride does not react with CuCN even at the boiling temperature and likewise no reaction is obtained with KCN at a temperature between 65.degree. and 130.degree. C (C. D. Hurd, O. E. Edwards, J. R. Roach, J. Amer. Chem. Soc. 66 (1944), 2014). With pivaloyl chloride and CuCN to be sure it is possible to obtain a reaction but the reaction time of 20 hours is extremely long (N. Sperber, R. Fricano, J. Amer. Chem. Soc. 72 (1950), 2792).
It is also known to react certain aliphatic carboxylic acid nitrile with CuCN in boiling acetonitrile but the yields are very small. Thus, starting from acetyl chloride there is isolated only 50% of acetyl cyanide and from pivaloyl chloride only 16% pivaloyl cyanide (Normant, Bull. Soc. Chim. France, 1972, pages 2402-2403). Somewhat higher yields of benzoyl, p-nitrobenzoyl cyanide and p-methoxybenzoyl cyanide are shown as well as valeroyl cyanide.
Aroyl cyanides are somewhat more easily produced from aroyl chlorides and metal cyanides, but the reaction conditions are still very disagreeable. For example, in the production of p-methoxybenzoyl cyanide from p-methoxybenzoyl chloride there is used mercury cyanide and a temperature range of 125.degree. to 130.degree. C (L. Rosenthal, Berichte deutsch Chem. Gesell. 44(1911), 2465).
A disadvantage of all of these processes is that there must be used an expensive heavy metal cyanide in at least stoichiometric amounts. If alkali cyanides are used there is preferentially obtained dimeric acyl cyanides (J. Thesing, D. Witzel, Angew. Chem. 68 (1956), 425).
It is also known that under vey mild conditions, for example, working at 0.degree. C in a two phase system, there can be obtained small amounts of aroyl cyanides and large amounts of dimerized acyl cyanides (Koenig, Tetrahedron Letters No. 26 (1974) pages 2275-2278).